Messages sent from a shared mailbox aren't saved to the Sent Items folder

Symptoms

Assume that you're using Microsoft Outlook 2016 or a later version, and you've been delegated permission to send email messages as another user or on behalf of another user from a shared mailbox. However, when you send a message as another user or on behalf of the user, the sent message isn't saved to the Sent Items folder of the shared mailbox. Instead, it's saved to the Sent Items folder of your mailbox.

Cause

In Microsoft 365, shared mailboxes don't require a license and can't be added to Outlook as independent mailboxes. You can't sign in to a shared mailbox. Instead, you sign in to your own mailbox, and then you open the shared mailbox. When you send or reply to a new message from the shared mailbox, Outlook automatically sends or replies from the sender's account. Therefore, messages are stored in the Sent Items folder of the sender's mailbox.

Workaround

To work around this issue, use one of the following methods.

Note: If you're using new Outlook, only Method 1 will apply.

Method 1

Configure the mailbox to save a copy of the message to the Sent Items folder of the shared mailbox in Exchange Online or in on-premises Exchange Server.

For emails sent as the shared mailbox, run the following command in Exchange PowerShell:

set-mailbox <mailbox name> -MessageCopyForSentAsEnabled $True

For emails sent on behalf of the shared mailbox, run the following command in Exchange PowerShell:

set-mailbox <mailbox name> -MessageCopyForSendOnBehalfEnabled $True

Method 2

Important

This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For protection, back up the registry before you modify it so that you can restore it if a problem occurs. For more information about how to back up and restore the registry, see How to back up and restore the registry in Windows.

Set the DelegateSentItemsStyle registry value on the Outlook client.

Note

For this option to work correctly, Outlook must be configured to run in Cached Exchange mode. For more information, see Email remains in the Outbox when you use the DelegateSentItemsStyle registry value.

Use the following steps:

1. Start the registry Editor.

2. Locate and then click the following registry subkey:

   HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences

3. On the Edit menu, point to New, and then click DWORD 32-bit Value.

4. Type DelegateSentItemsStyle, and then press Enter.

5. Right-click DelegateSentItemsStyle, and then click Modify.

6. In the Value data box, type 1, and then click OK.

7. Exit Registry Editor.

More information

The following table lists the expected behavior based on different combinations of the DelegateSentItemsStyle registry value setting and the mailbox's MessageCopyForSentAsEnabled parameter value.

DelegateSentItemsStyle	MessageCopyForSentAsEnabled	Expected behavior
0	True	A copy of the email will be saved in both the primary mailbox and the shared mailbox.
1	True	Two copies of the email will be saved in the shared mailbox and no copies in the primary mailbox.
0	False	A copy of the email will be saved in the primary mailbox and no copies in the shared mailbox.
1	False	A copy of the email will be saved in the shared mailbox and no copies in the primary mailbox.

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